In order to make a hollow-wall container, for instance for marketing yoghurt or the like, it is standard as described in German patent 4,002,503 filed 29 January 1990 by H. Strunk to use a mold that has two or three parts that define a die cavity and that also has a core element that is advanced into the die cavity and that is only retracted when the resin has substantially cured after the shot. The result is a hollow formed in the molded article that provides structural rigidity, makes the package look bigger than it really is, and that even offers some thermal insulation.
The disadvantage of this system is that it is normally necessary for sanitary or other reasons to cover the mouth of the hollow formed by the retracted core element. Thus in a separate step a separately cast piece is fitted over the mouth of this hollow. Clearly this extra step increases production costs.